Fresh water corrosion and anode choices…

[Lou C]

Something there is VERY wrong! Never seen this in 20 years of salt water use, not just use, but mooring an outdrive in salt water.
You need a marine electrical expert.

 

[dingbat]

An electric leak in the lake seems unlikely but I am going to test for it.

Generally speaking, aluminum doesn’t corrode in freshwater w/o facilitation.

The “electric leak in the lake” may in fact be a short or bad ground connection somewhere on the motor or boat.